1. Field of the Invention
The present invention relates to the communication of digitally encoded data, and in particular to systems in which the data is scrambled in accordance with a binary sequence before transmission. As described below, the present invention has been found to be particularly advantageous in connection with the encoding of video signals but is also applicable to different forms of data.
2. Related Art and Other Considerations
It has previously been proposed to scramble encoded video data prior to transmission by combining at an XOR gate the outputs of a coder and a PRBS (pseudo-random binary sequence) generator. The scrambled data can then be read only at a receiver having a matching descrambler. The coder may encode the input signal using, for example 5-bit hybrid DPCM (differential pulse code modulation) as described in the paper by M. C. W. Van Buul, "Hybrid D-PCM, A combination of DPCM and PCM", IEEE Trans. on Communications, Vol. COM-26, No. 3, 3/78 pp 362-8. The parallel 5-bit codewords output by such a coder are converted to a serial bit-stream before being scrambled by the pseudo-random code sequence. Then at the receiver the serial data is unscrambled using a matching PRBS generator before being assembled into 5-bit codewords which are decoded to reconstitute the original video signal.
In order to descramble the data correctly it is necessary that the PRBS generator in the receiver should be aligned with that in the transmitter so that corresponding bits in the pseudo-random sequence are applied to the same elements of data in the scrambler and descrambler. As described in the present applicant's earlier British applications numbers 8804552 and 8721603, this is achieved by searching for a video line synchronisation or frame alignment word, or other predetermined fixed component of the received data and aligning the PRBS generator accordingly. While this is sufficient to ensure appropriate descrambling of the serial data it is also necessary to ensure that the conversion of the serial data to parallel 5-bit words is carried out in correct alignment with the transmitter to avoid corruption of the data. This may be achieved by adding extra synchronization bits to the video data but this gives rise to an undesirable overhead in the transmission rate. Alternatively it has been proposed to replace some of the least significant bits of the video signal with synchronization words. This however introduces small but regular errors into the video signal which may be visible when that signal is reconstituted.